The present invention relates to a method of simultaneously transmitting a plurality of digital data streams over one channel. More particularly, the present invention relates to a method of providing the synchronizing signal for a data channel in which a plurality of digital data streams are being transmitted by means of modems which does not require the transmission of additional digital synchronizing information.
If a plurality of data streams are to be transmitted simultaneously over one channel, this can be done by means of digital multiplexers. However, in every case this requires additional digital information which reduces the effective transmission rate. If the transmission is effected with modems in which modulation takes place in groups of bits (e.g. as disclosed in CCITT Recommendations V26, V27 and V29, Vol. VIII.1, pages 101-112, 118-139 and 143-153, respectively), the bit groups can be split into individual channels. No special additional information is then required. For example, in a modem according to CCITT Recommendation V29, four bits are combined into a bit group (quadbit) and the first bit can then be assigned to channel 1, the second bit to channel 2 etc. The result is that there are again four channels with standard bit rates. However, the desire is to transmit data streams at rates which cannot be combined into bit groups.
This problem arises, for example, if two channels such carrying 2400 bit/s are to be transmitted via a modem with 4800 bit/s according to CCITT Recommendations V27 bis/ter. According to that proposal, 3 bits are combined into tribits and are modulated into eight different phase shifts with 1600 baud.